There are several choices available as a filling material for filling a dental cavity: a traditional amalgam (a silver-mercury alloy), a glass ionomer, and a composite resin. In the past two decades, composite resin filling material has become the material of choice for dentists for filling dental cavities due to their exceptional esthetic properties, i.e. precise color-matching capability and excellent translucency. However, current composite resins generally do not have a self-adhesive property and generally require pre-treating the tooth structure (or tooth cavity wall) with an adhesive to bond the composite to the tooth structure. For some adhesives, an etchant and/or a primer are needed to etch and/or condition the tooth structure prior to adhesive application. For a 4th generation dental adhesive system, such as OptiBond® FL (Kerr Corporation, Orange, Calif.), the restorative procedures would include the following steps: (1) remove carious dentition (dentin and enamel); (2) etch the cavity wall with a 37% Phosphoric Acid Gel Etchant (Kerr Corporation, Orange, Calif.); (3) rinse the etchant off the cavity wall with water; (4) briefly dry the cavity wall with compressed air using a dental air-syringe; (5) apply a layer of OptiBond® FL Primer (Kerr Corporation, Orange, Calif.) to the cavity wall; (6) evaporate the solvent within the primer with compressed air using a dental syringe; (7) apply a layer of OptiBond® FL Adhesive (Kerr Corporation, Orange, Calif.) to the primer-coated cavity wall; (8) air-thin the adhesive with compressed air; (9) light-cure the adhesive with a dental curing light; (10) place a composite resin filling material, such as Premise™ (Kerr Corporation, Orange, Calif.), either in bulk (or single increment) or in several increments; (11) light-cure the composite resin of each increment; and (12) remove excess composite resin and polish the restoration. Because of the many components and steps involved with the current dental cavity filling procedure using a composite resin filling material, the restorative procedure is quite complicated and time-consuming. Also, the procedure could be quite technique-sensitive as each specific adhesive has its own unique application procedure and care has to be given for each step for ensure a successful restoration. In addition, because of the many materials and steps involved, there will be increased chances for mistakes (i.e. using the wrong material or sequence of materials) and also contaminations due to saliva or blood during the procedure.
Recent development in the adhesive arena has simplified the restorative procedure to some extent with the introduction of various new primer and/or adhesive configurations: total-etch priming adhesive (the primer and adhesive are combined into a single part), self-etch primer (the etchant and primer are combined into a single part), and a self-etch adhesive (the etchant, primer and adhesive are combined into a single part). Nevertheless, an adhesive or an adhesive in combination with either an etchant or a self-etch primer is still needed to bond the composite resin filling material to the cavity wall.
It is highly desirable to further simplify the cavity-filling restorative procedure by imparting self-adhesiveness to the composite resin filling material and therefore eliminate the need for an etchant and an adhesive. To formulate a self-adhesive composite resin, adhesive monomers need to be incorporated into composite resin to provide an adhesive property to the composite resin. However, most adhesive monomers have high polymerization shrinkage due to their low molecular weight. The adhesive monomers generally have a high viscosity and tend to limit the amount of inorganic fillers that can be incorporated into the composite resin, resulting in more polymerization shrinkage. Therefore, a self-adhering composite resin incorporating adhesive monomers is expected to have excessive polymerization shrinkage upon light-curing, exerting excessive shrinkage force at the bonding interface between the composite resin filling material and the cavity wall. The excessive shrinkage force can cause debonding and therefore microleakage at the bonding interface, leading to secondary caries later on. Another potential issue with a self-adhering composite resin filling material is that the material is expected to be quite sticky due to the incorporation of adhesive monomers and can further compromise the bonding interface due to excessive stickiness. Another issue with a self-adhering composite resin filling material is that the adhesiveness of the material would be compromised due to much increased filler loading and therefore viscosity, making it difficult to effectively wet the tooth surface to establish a reliable bond without the help of an etchant and/or an adhesive. For those reasons or issues, there is still no viable commercialization of a truly self-adhering composite resin filling material that would eliminate the need for an etchant and/or an adhesive.